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Martian gas If there is life deep below the surface of Mars, we may be able to detect it simply by measuring the methane in the planet's atmosphere, say researchers.

Dr Chrisopher Oze, a geologist at New Zealand's University of Canterbury, and colleagues investigated the effects that living things have on the production of methane in hot, wet environments. Their findings are published this week in the journal PNAS.

On Earth, most of the methane in the atmosphere is produced by living things. The gas is also produced by geothermal processes, such as volcanic eruptions and decaying coal.

In the new experiments, researchers replicated conditions in hydrothermal vents on the ocean floor of Earth and five kilometres below the surface of Mars. Under these conditions, a common mineral called olivine is altered through a process called hydrolysis that eventually results in the production of methane.

"Olivine is the most abundant mineral in our planet's crust," said Oze. "On Mars, scientists are also finding more and more of it on the surface, which means that there's likely to be more of it under the surface too."

In the experiments, which included no living things, the researchers monitored the ratio of hydrogen gas to methane for up to 861 hours. Eventually, the levels of the two gasses settled down to a near constant hydrogen-to-methane ratio of 42.

Next, the researchers looked at other studies that measured the ratio of hydrogen and methane and found good agreement with their own findings. In other lab experiments that excluded living things, the lowest hydrogen to methane ratio reported was also 42.

On the other hand, in natural settings where methane production was affected by living organisms, the ratio of the two gasses never exceeded 33. And in most cases, the ratio of hydrogen to methane in these living systems was less than 10.

"In the studies we looked at there were no exceptions to the rule," says Oze. "This is a significant finding that demonstrates that the hydrogen to methane ratio may be used to evaluate if life is present."

"Mars has all the same components that we included in this study," he explained.

"If life is similar on Mars to Earth, then we can use the ratio of hydrogen to methane in the atmosphere to peel back the surface of the planet and see whether there is life under there."